The transconductance (Gm) of a cell is defined as the ratio of the output current IOUT to the input voltage VIN. The transconductance of a Gm cell on an integrated circuit varies over process and temperature. Depending on the on-chip components used for the cell, this variation can be greater than ±30%.
This variation is unacceptable for integrated circuits which rely on a constant transconductance. For example, a constant transconductance is required in order to keep the gain constant in low noise amplifiers (LNAs) having inductive collector loads, or to minimise the variation in the start up gain of a voltage controlled oscillator (VCO). In another example, a constant transconductance is required to minimise frequency error in a constant GmC-filter, such that the variation in the frequency characteristic of the filter frequency is reduced to that due to variation in the capacitors. The variations in capacitance values are usually small compared to the variations in transconductance.
In GmC-filters and gyrator-C circuits, it is known to use an “autotuner” for controlling the tuning frequency of the filter.
These circuits use a secondary filter or oscillator using a similar Gm C cell within a control loop, where the frequency of the filter is compared with an external frequency reference to generate an error signal. GB1,421,093 is an example of a GmC-filter having an external frequency reference. U.S. Pat. No. 6,304,135 is another example, in which the GmC-filter uses an external resistor for controlling the frequency. U.S. Pat. No. 4,388,539 discloses a constant Gm circuit which, as above, has the disadvantage of requiring the use of a dedicated external resistor for maintaining the transconductance constant.
Thus, all of the known solutions have the disadvantage of requiring an extra external frequency reference or a dedicated external resistor to achieve a constant transconductance, the external frequency reference or resistor being used to fine tune the filter response. The need for an external frequency reference or resistor also has the disadvantage of requiring one or more extra pins on the integrated circuit.
The aim of the present invention is to provide an integrated circuit having a constant Gm biasing circuit which does not have the disadvantages mentioned above.